It is sometimes necessary to move an aircraft in reverse whilst the aircraft is on the ground. For example aircraft are frequently pushed backwards away from an airport gate, a so called “pushback” manoeuvre, by a vehicle known as a pushback tractor or tug. The same tractor or tug can also move the aircraft forward if desired. Whilst some aircraft have the capability to reverse under their own power using the main aircraft engines, e.g. by using reverse thrust in a manoeuvre known as “power back”, this is not permitted for civil aircraft as the jet or prop wash from the engines can cause damage to nearby terminal buildings or facilities.
Recently it has been proposed to equip aircraft with an autonomous wheel drive taxi system that can drive one or more of the aircraft landing gear wheels in rotation so that the aircraft can taxi under its own power. The ability to drive the aircraft landing gear wheel in reverse using the wheel drive system enables an autonomous pushback operation without the main engines running, as well as a forward taxiing operation with or without one or more of the main engines running. The autonomous pushback operation makes the autonomous wheel drive taxi system particularly suitable for civil aircraft but the system has wide applicability to a wide variety of aircraft including: —civil and military; fixed wing, rotary wing and powered lift; manned and unmanned, etc.
For conventional pushback operations, braking is performed by the tractor. Use of the brake pedals is generally prohibited as there is a risk of damaging the nose landing gear and the tractor. With an autonomous wheel drive taxi system, i.e. without a tractor unit, braking will need to be performed autonomously on the aircraft. Conventional aircraft braking systems are typically not designed to perform this function, being more suited to high energy dissipation in the landing phase and low energy dissipation working against the thrust of the main aircraft engines in the taxi phase. Conventional aircraft braking systems are therefore generally unsuited to braking during an autonomous pushback operation. The autonomous pushback introduces the risk of aircraft tip over (where the aircraft tends to rotate nose up about its pitch axis, possibly causing a tail strike) and aircraft runaway (where if the autonomous pushback manoeuvre is performed on a slope the longitudinal component of the gravity acceleration can become higher than the rolling resistance of the tyres on the ground such that the aircraft accelerates above the desired pushback speed).